This invention is in the field of light curing units employed in the polymerization of photoreactive resins. 2. The Relevant Technology
Light activation units, also known as light curing units, are employed to polymerize photoreactive resins in a variety of industries. Light activation units typically include a light source having a certain intensity and a timer which controls the illumination time during which the light source emits light energy.
Typically, before actuating the light curing unit, the practitioner measures the intensity of the light source, then manually sets the timer for a desired illumination time. Upon actuating the unit, the timer causes the unit to emit light for the selected illumination The total light energy emitted per area of resin is the product of the intensity of the bulb multiplied by the illumination time. In other words, the total light energy emitted is the product of the wattage of the bulb multiplied by the illumination time.
In the field of dentistry, for instance, light curing units are often employed to polymerize photoreactive resins, such as light curable resins, composites, and other polymers containing photoinitiators. By way of example, a photoreactive resin is often employed to attach a dental appliance such as a veneer to a dental surface. The resin is disposed on the veneer, after which the veneer is placed against the tooth. The light curing unit is then directed toward the translucent veneer and actuated for a selected illumination time, emitting a total light energy into the resin. The light energy polymerizes the resin, maintaining the veneer firmly in place.
Typical light curing units are designed to emit a total light energy such that the photoreactive resin is polymerized, transforming the photoreactive resin from a liquid state to a solid state. These units are typically designed so that light curing units emit light in increments of tens of seconds. Thus, the practitioner only has the option of setting the time to produce an illumination time of ten seconds, twenty seconds, thirty seconds, and so on up to sixty seconds. As a result, the practitioner does not have the option of fine tuning the illumination time to transform the photoreactive resin from the liquid state to a desired state intermediate to the liquid state and the solid state.
One difficulty with these typical light curing units is that once a photoreactive resin is in the solid state, it is difficult to remove excess resin disposed on a dental surface without damaging the underlying dental surface or the dental appliance attached by the resin. In addition to causing unsightly physical appearance, excess resin disposed on a dental surface can cause a deformed tooth structure, interfering with the smooth functioning of a particular dental surface and/or cause soft tissue problems.
When excess resin is applied to bond the dental appliance to the tooth, which often occurs during dental restorations, the practitioner may be required to damage the dental appliance or damage the underlying dental surface in order to remove the excess. Even if the dental appliance or dental surface is not damaged during removal of the excess, the practitioner may be required to grind off hardened resin. The grinding, breaking, or scraping of hard material required to clean off the excess resin is labor intensive.
Furthermore, even in the liquid state, photoreactive resins are difficult to remove from a dental surface. If a liquid resin is placed on a surface, it often leaves a resin film despite various attempts to wipe off the liquid resin.
Another difficulty with typical light curing units is that light curing units are also employed to polymerize photoreactive flowable type resins disposed on or within a tooth as filling material. Because of the liquid nature of these resins, the resins often migrate from the desired surface, flowing out of a hole drilled in a tooth, for example. In an attempt to prevent undesired migration, practitioners have constructed a mold surrounding the tooth designed to retain liquid resin in a desired area.
Although it is possible for a practitioner to actuate a typical light curing unit and manually turn the unit off when a desired illumination time has been reached, thereby achieving a resin stage intermediate the liquid and solid stages, the practitioner is required to simultaneously view a clock or otherwise keep track of the time or the nature of the resin during the procedure, which is inconvenient while working inside a patient's mouth, for example. Furthermore, it is difficult, if not impossible to accurately, consistently, reproduce such a manual procedure. In addition, the desired illumination time may be in the tenths or hundredths of seconds, compounding the difficultly of accurately, consistently reproducing the total energy provided to a particular polymer when using a manual procedure.
Another difficulty within the art is that the light source of a typical light curing unit often experiences variations in light intensity. These variations are typically decreases in intensity caused by decreases in voltage, amperage, or by deteriorating bulb condition. For example, tungsten from the filament of a light bulb can be deposited on the inside of the bulb, decreasing the intensity of the bulb. Over time, decreases in intensity become more noticeable.
Variations in intensity make it even more difficult for the practitioner to provide a total light energy to a photoreactive resin which will cause the resin to achieve an intermediate stage. Typically, the practitioner must compensate for these variations by manually increasing the illumination time or by replacing a light bulb. However, it is uneconomical to replace a light bulb whenever a decrease in intensity occurs. Furthermore, monitoring the intensity of the bulb and manually adjusting for intensity variations is cumbersome.
Another problem within the art relates to the continuous nature of typical light curing units. Typical light curing units provide illumination without interruption. It is believed that when photoreactive resins are cured on a continuous basis, the photoreactive resins suffer from polymeric shrinkage, stress and strain. Shrinkage, stress, and strain result in a weaker bond and/or seal or potentially converts to stress on teeth.